1. Field of the Invention
This invention relates in general to DC motors and in particular to a novel DC motor control circuit.
2. Description of the Prior Art
FIG. 1 illustrates a conventional brushless DC motor drive circuit wherein reference characters L1, L2 and L3 designate stator windings of a brushless DC motor which are connected in Y connection and spaced 120.degree. from each other.
A rotor Rt which is a permanent magnet is rotated inside of the windings L1, L2 and L3 and the rotational positions of the rotor Rt is detected by position detectors H1, H2 and H3 which are mounted around the rotor Rt 120.degree. from each other.
The position detectors H1, H2 and H3 are Hall devices for example and produce position signals P1, P2 and P3 which are supplied to a drive pulse generator 1 which generates switching pulses which are supplied to output transistors Q1 through Q6 connected as shown. A motor drive voltage Vm is supplied to a power source terminal 2 and is sequentially applied to motor windings L1, L2 and L3 through the transistors Q1 through Q6 from drive pulse generator 1. FIG. 2A illustrates a set of positions signals P1, P2 and P3 which are generated by the Hall devices H1, H2 and H3, respectively. FIG. 2B illustrates the direction of the motor currents which flow through two of the three windings at any one time and as the rotor Rt rotates the conditions are switched to the next state as shown. As the rotor Rt rotates, reverse electromotive forces E1, E2 and E3 are generated in the stator windings as illustrated in FIG. 2D. FIGS. 2A through 2D illustrate wave forms for rotating the rotor Rt in the clockwise direction and if a direction control pulse is supplied at terminal 3 and when it changes the sequence of the switching pulses are changed on the transistors to cause the rotor Rt to rotate in the counterclockwise direction. If the brushless motor is installed in a video tape recorder, for example, as a reel drive motor, the brushless motor is required to be controlled from a speed of 100 rpm in the reverse direction to 100 rpm in the forward direction linearly. Thus, the tape speed control requires linear control of the motor drive torque as well as the motor braking torque.
In prior art driving circuits such as shown in FIG. 1, transistors Q.sub.1 through Q.sub.6 are considered to be diodes Dn and Dn+1 because the transistors Q.sub.1 through Q.sub.6 are unidirectional devices. The equivalent circuit of the driving circuit during driving operation is illustrated in FIG. 3A. An equivalent circuit during braking operation is illustrated in FIG. 3B. In these drawings, the resistance Rm represents the resistance of the windings and Im represents current flowing through the windings.
The driving torque of the motor is generated when the current Im flows in the opposite direction to the reverse electromotive voltage En. In this case, the relationship of the absolute value of Vm must be greater than the absolute value of En. On the other hand, a braking torque is generated when the current Im flows in the same direction to the reverse electromotive voltage En in case of reverse drive braking.
As is illustrated and is apparent from the equivalent circuit of FIG. 3B when the source voltage Vm is zero, the current Im flows through the windings corresponding to the reverse electromotive voltage En and a corresponding torque is generated which is proportional to the current Im and it is very difficult to control the torque of the motor in a linear fashion over a wide range.
So as to eliminate this problem, it is proposed to provide additional diodes D'n and D'n+1 as illustrated in FIG. 4 so as to establish a reverse current path. If the relationship Vm is less than En, the current flowing through the diodes D'n and D'n+1 could generate a regenerated braking torque.
In such a construction, a switching control between the reverse driving brake and the regenerated brake is required based on a rotational speed of the rotor Rt and such switching circuit is very critical in operation.